1. Field of the Invention
The present invention relates in general to a method for removing pollutants from a flue gas stream, and in particular, to a method and apparatus for removing acid gases, air toxics and trace metals from a flue gas stream formed during the combustion of waste materials, coals, and other fossil fuels burned in either electric power generating plants, steam generating plants, waste-to-energy plants, or other industrial processes.
2. Description of the Related Art
Removal of pollutant emissions from fossil fuels such as coal, oil, petroleum, coke, orimulsion, etc. and/or waste materials such as municipal waste, tires, etc. is a major concern of electric power generation plants, waste-to-energy plants, and other industrial processes. Prior art attempts to reduce or limit these pollutant emissions have included the following techniques:
1. Locating and utilizing fuels lower in content of sulfur, chlorides, heavy metals and/or other contaminants. PA0 2. Reduction of sulfur content, chloride content, heavy metals content, and/or other contaminants in the fuel by separating various materials from the fuel stream. The major disadvantage with this approach is the cost effectiveness of separation required to achieve the desired levels of reduction. PA0 3. Minimizing the formation of volatile organic/chlorinated organic compounds in the combustion process, by increasing gas temperature and/or gas residence time in the furnace. This approach is not always sufficient to limit the levels of organic or chlorinated organic compounds and other contaminants to the required levels. PA0 4. Spray drying chemical absorption processes, i.e., dry scrubbing, wherein an aqueous alkaline solution or slurry is atomized (via mechanical, dual fluid or rotary atomizers) and sprayed into a hot flue gas to remove sulfur oxides and other contaminants. Additionally, the spray drying process can cause condensation of volatile organic or chlorinated organic compounds and/or heavy metals. These condensed compounds can become attached to solid particles in the flue gas stream and subsequently removed with solid particles in the particulate collector device downstream. The disadvantages with this approach include high pressure losses, significant real estate requirements and capital cost of the dry scrubber vessel. PA0 5. There exist activated carbon technologies which have been used as a method of removing various pollutants, including volatile organic/chlorinated organic compounds and heavy metals from flue gas streams. A typical approach is to inject activated carbon into the flue gas stream prior to a dry scrubber and baghouse. The activated carbon functions as an adsorbent for a variety of pollutants and is subsequently removed from the gas stream in the downstream fabric filter bags of the baghouse. A disadvantage with this approach is the additional capital cost, activated carbon disposal and replacement costs; and operating cost, above the initial investment, and operating cost of the dry scrubber system. PA0 6. Wet chemical absorption processes (i.e., wet scrubbing) wherein the hot gas is typically washed in a countercurrent flow gas/liquid contact device with an aqueous alkaline solution or slurry to remove sulfur oxides and other contaminants. The major disadvantages with this approach are the loss of liquid both to the atmosphere (i.e., due to saturation of the flue gas and mist carry-over) and with the sludge produced in the process, the economics associated with the materials of construction for the absorber module itself, and all related auxiliary equipment downstream (i.e., primary/secondary dewatering and waste water treatment subsystems).
Several examples of patents showing these approaches include U.S. Pat. Nos. 4,696,804; 4,632,810; 4,366,133; 4,623,523; and 4,980,099.
A need remains to improve the removal of pollutant emissions from fossil fuels and waste materials particularly for acid gases including SO.sub.x, HCl, and air toxics (volatile organic or volatile chlorinated organic compounds) and trace heavy metals.